The production of dentures in the dental laboratory usually takes place using a working model which reproduces the tooth and jaw ratios of the patient as faithfully as possible. The dentist in attendance initially takes a negative impression using elastic molding materials which are distinguished by high molding accuracy, high form-retention and good reproduction of details. Such molding materials are, for example, the so-called reversible hydrocolloids of an agar-agar base, irreversible hydrocolloids of an alginate base, polysulfides, polyethers and silicones cross-linked by condensation and addition.
Dental impressions, which are also known as negative molds, thus obtained can then be filled with a molding material by the dentist or dental technician and thus provide the working model. As the molding materials, plasters in particular are available which can differ greatly with regard to their quality and price. Here, it is of course true that the higher-quality plasters, which also have a good surface-finish, are comparatively expensive.
Attempts have therefore already been made to improve models produced from cheaper types of plaster by surface treatments. This can be effected, for example, by the application of a hardenable lacquer. This is not however a satisfactory solution as a rule, because the hardening coating allows only poor adhesion to the plaster and in addition the accuracy of the model material is affected by the coating applied.
A further problem related to the surface quality of these plasters is the negative effect of the surface-active substances which can be present on the surface of the negative molds. The use of these surface-active substances, or surfactants, is necessary if the negative mold has to be thoroughly cleaned, disinfected or its surface tension reduced. The surface-active substances become concentrated on the surface of the negative molds, made, for example from hydrocolloids and despite rinsing cannot be completely removed. This leads ultimately to an impairment of the surface quality of the plaster and therefore to an impairment of the molding accuracy of the plaster model.
It is known from "The Science of Dental Materials", 5th edition, 1960, W.B. Saunders Company, Philadelphia and London, to add, inter alia. fluorine compounds to the molding materials to increase the surface hardness of the plaster molds so produced (see page 94). The detrimental effect of the surface-active substances which adhere to the impressions is not however influenced by this. When cheaper types of plaster are employed, even if molding materials containing fluorine compounds are used, one ultimately obtains plaster models which have sandy surfaces and whose contour definitions leave much to be desired.